It is known to execute a brake assist control so as to generate a higher braking force than usual by a normal braking action when the driver depresses the brake pedal quickly when a vehicle such as an automobile is running. In a vehicle equipped with a common fluid (oil) hydraulic brake system, the brake assist control is executed such that the master cylinder pressure varying according to the depression of the brake pedal by the driver is increased by a pump before being supplied to the wheel cylinders of the respective wheels, or an oil pressure generated by a pump separately from the master cylinder is supplied to the wheel cylinders of the respective wheels, so that thereby a strong braking force is generated corresponding to an increased oil pressure in the wheel cylinders. The quick braking action is detected by, for example, the depressing speed of the brake pedal. According to such a brake assist control, it is possible, even when the depressing force by the driver is insufficient, to generate a strong braking force for swiftly decelerating or stopping the vehicle.
During the execution of such a brake assist control, since the braking forces of the respective wheels increase quickly, the wheels are liable to locking. When the slip ratio of any wheel becomes excessive by the execution of the brake assist control, an anti-skid control (ABS) is executed so as to suppress the braking force from becoming excessive, as described in, for example, Japanese Patent Laid-open Publications Hei 9-290746 and Hei 10-59149 based upon the applications filed by the same applicant as the present application. (In Japanese Patent Laid-open Publication Hei 9-290746, there is described a brake assist control in a case that a vehicle performance modification device such as the anti-skid control does not normally operate. According to this publication, in consideration that the rear wheels are liable to locking prior to the front wheels, the gradient of the braking pressure increase of the rear wheels by the brake assist control is decreased so as to prevent that the side force of the rear wheels lowers rapidly.)
Further, when the wheels become liable to locking due to an increase of the braking force by the brake assist control, there sometimes occurs that other control such as the vehicle behavior stability control (VSC) or the braking force front/rear distribution control (EBD) do not effectively operate. In view of this, in the above-mentioned Japanese Patent Laid-open Publication Hei 10-59149, for example, it is proposed that, when an oversteer tendency is to be suppressed, the brake assist control of the rear wheels is prohibited or the increase of the braking force by the brake assist control is suppressed, so as to prevent a too much lowering of the cornering force of the rear wheels due to an increase of the braking force by the brake assist control.
As briefly described above, when the brake assist control is executed in the oil-hydraulic brake system, the braking root pressure (the pressure in a common passage for conducting the oil pressure before being branched toward the respective wheel cylinders) is increased by a pump, so that the increased braking root pressure is supplied to the respective wheel cylinders to increase the braking pressures therein. When the anti-skid control is executed with respect to a certain wheel in such a condition, a valve provided for the wheel cylinder of a particular wheel is operated to restrict the flow of the brake oil to the wheel cylinder. In other words, when a braking force increase suppression control such as the anti-skid control is executed to suppress the increase of the braking force during the execution of a braking force increasing control such as the brake assist control, there occurs a control which is inefficient in the energy consumption such that the braking root pressure is increased in the brake system on one hand, while the pressure is suppressed in the same brake system on the other hand. The same situation occurs when the vehicle behavior stability control to selectively increase or decrease the braking pressure or the braking force front/rear distribution control is executed during an execution of the brake assist control.
Nevertheless, the energy loss in such a parallel execution of the braking force increasing control and the braking force suppressing control in the same brake system was scarcely considered in the conventional braking force control.